Switching between network based and relay based operation for mission critical voice call

ABSTRACT

A device, serving cell and method of switching between modes of operation for the device in a cellular network comprises receiving a service via a connection with the serving cell. A condition indicative of deterioration in network service and an absence of a suitable neighbor cell are detected. In response, discovery of relay nodes is initiated. When a suitable relay node is discovered, a mechanism to switch to receiving the service via the suitable relay node is initiated. The service may be a mission critical push-to-talk over Long Term Evolution service. The relay node may be user equipment acting as a user equipment-to-network relay node. When the device is in an RCC_CONNECTED state, the mechanism to switch includes sending a relay mode preference indication to the serving cell, receiving a connection release message from the serving cell, releasing the serving cell connection, and establishing a sidelink connection with the relay.

BACKGROUND Technical Field

The present disclosure relates to the use of Proximity-based Services(ProSe) User Equipment-to-Network Relays (UNRs) in the scope ofdevice-to-device communication and more particularly to servicecontinuity for a ProSe-enabled device switching back and forth between amode of operation by which the device gains access to services directlyfrom the cellular network and a mode of operation by which the devicegains access to services through a UNR.

Description of the Related Art

In general, ProSe capable devices (e.g., devices that support ProSe andMission Critical Push-To-Talk (MCPTT)) are expected to be mobile. Alsonetwork coverage may not be available everywhere. Thus, User Equipment(UE) (e.g., mobile devices such as cellular phones, tablets, laptopcomputers, etc.) move in and out of network coverage. When in coverageof the network, a UE can receive the services provided by the network inthe Network Mode Operation (NMO), wherein the network directly providesthe services such as MCPTT over Long Term Evolution (LTE) service toMCPTT UEs that are within radio coverage of an evolved node B (eNB)(i.e. an LTE base station). Whilst out of coverage of the network, aProSe UE within coverage of a UE-to-Network Relay (UNR), a node relayingthe services from the network to other UEs, relies on this UNR formaintaining access to the MCPTT services and may enter Network ModeOperation via Relay (NMO-R) for this purpose.

In some instances, a UE may experience a service interruption andunacceptable latency (e.g., for the Public Safety services) when movingfrom a location where there is network coverage (NMO) to a locationwhere there is no network coverage whilst in coverage of a UNR (i.e.with NMO-R opportunity). Specifically, the interaction between the eNBand the UE or the eNB and the UNR in order to facilitate this switchbetween NMO and NMO-R has not been specified in detail and it remainsopen how these mechanisms will be achieved at the radio level.

As an example, subsequent to the disconnection from an LTE network dueto a Radio Link Failure (RLF) and before the UE would recover servicethrough a relay, there may be a significant service interruption delay.This delay is in the order of 0.5 to 18 sec and such a delay may be aproblem for most of the mission critical services which are beingtargeted by 3rd Generation Partnership Project (3GPP).

Similar to the above situation, a UE operating in NMO-R may need totransition from NMO-R to NMO without a large service interruption time.For example, remaining connected via a UNR, when network coverage isavailable may result in excessive battery consumption by the UNR andreduced quality of service. It is also generally accepted that UEbehavior be predictable and deterministic in such scenarios.

Existing seamless mobility provided by handover mechanisms areunsuitable/inappropriate because the serving cell may not be aware ofthe existence of a suitable relay, and/or may not be able to initiate ahandover preparation phase with the relay. Connected mode mobility toClosed Subscriber Group (CSG) cells relies on UE-based discovery andnetwork-based preparation phase followed by a handover, which is notpossible because handover of a UE to a UNR is not feasible. Autonomousreselection while in connected mode, while permitted in Universal MobileTelecommunications System (UMTS), Global System for MobileCommunications (GSM) and General Packet Radio Service (GPRS) (i.e. inpacket transfer mode), is not allowed in LTE. In any case, mechanismsusing the existing neighbor cell discovery cannot be used for autonomousreselection to UNR because these will result in higher powerconsumption.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute partof this specification, illustrate embodiments of the present disclosureand together with the description, serve to explain the principles ofthe disclosure. The embodiments illustrated herein are for examplepurposes only and not limited to the precise arrangements andinstrumentalities shown, wherein:

FIG. 1 is a block diagram of an example system for providing coverageextension using ProSe User-to-Network Relays (UNRs) in accordance withone aspect of the present disclosure;

FIG. 2 is a flow chart illustrating an example process for triggeringthe switch of a User Equipment (UE) operating in Network Mode Operation(NMO) to Network Mode Operation via Relay (NMO-R) in accordance with oneaspect of the present disclosure;

FIG. 3 is a block diagram illustrating an association of various peerprotocol layers in NMO and NMO-R;

FIG. 4 is an example event flow diagram illustrating a userequipment-triggered process by which a UE operating in NMO in theRRC_CONNECTED state is switched to NMO-R using a Break-Before-Make (BBM)approach according to an aspect of the present disclosure;

FIG. 5 is an example event flow diagram illustrating a process fortransitioning NMO logical channels to NMO-R according to an aspect ofthe present disclosure;

FIG. 6 is an example event flow diagram illustrating a UE-triggeredprocess for switching a UE operating in NMO in the RRC_CONNECTED stateto NMO-R using a Make-Before-Break (MBB) approach according to an aspectof the present disclosure;

FIG. 7 is an example event flow diagram illustrating a UE-triggeredprocess for switching a UE operating in NMO in the RRC_IDLE state toNMO-R according to an aspect of the present disclosure;

FIG. 8 is an example event flow diagram illustrating a network-triggeredprocess for switching a UE operating in NMO in the RRC_CONNECTED stateto NMO-R using a MBB approach according to an aspect of the presentdisclosure;

FIG. 9 is an example event flow diagram illustrating a network-triggeredprocess for switching a UE operating in NMO in the RRC_CONNECTED stateto NMO-R using a BBM approach according to an aspect of the presentdisclosure;

FIG. 10 is an example event flow diagram illustrating a process forinterrogating one or more UNRs about their capacity to support anadditional incoming UE according to an aspect of the present disclosure;

FIG. 11 is an example event flow diagram illustrating a process forunconditionally switching a UE operating in NMO-R to NMO using a MBBapproach upon finding network coverage, according to an aspect of thepresent disclosure;

FIG. 12 is an example event flow diagram illustrating a process forunconditionally switching a UE operating in NMO-R to NMO using a BBMapproach upon finding network coverage, according to an aspect of thepresent disclosure;

FIG. 13 is a flow chart illustrating an example process for triggeringthe switch of a UE operating in NMO-R to NMO in accordance with oneaspect of the present disclosure;

FIG. 14 is an example event flow diagram illustrating a process forswitching a UE operating in NMO-R to NMO using a MBB approach uponsatisfying triggering conditions, according to an aspect of the presentdisclosure;

FIG. 15 is an example event flow diagram illustrating a process forswitching a UE operating in NMO-R to NMO using a BBM approach uponsatisfying triggering conditions, according to an aspect of the presentdisclosure;

FIG. 16 is a block diagram of an example UE in accordance with an aspectof the present disclosure; and

FIG. 17 is a block diagram of an example eNB in accordance with anaspect of the present disclosure.

DETAILED DESCRIPTION

Embodiments provide for a device, serving cell and method of switchingbetween modes of operation for the device in a cellular network. Inaccordance with one aspect of the present disclosure, a method isprovided for switching between modes of operation for a device in acellular network. A service is received via a connection with a servingcell. While receiving the service via the connection with the servingcell, a condition indicative of a deterioration in the service receivedvia the connection with the serving cell and an absence of a suitableneighbor cell are detected. Responsive to detecting both the conditionindicative of a deterioration in the service received via the connectionwith the serving cell and the absence of a suitable neighbor cell,discovery of relay nodes is initiated.

In accordance with another aspect of the present disclosure, a deviceoperating in a cellular network includes a communication subsystem thatreceives a service via a connection with a serving cell and a processor.The processor is communicatively coupled with the communicationsubsystem. While the at least one communication subsystem is receivingthe service via the connection with the serving cell, the processordetects a condition indicative of a deterioration in the servicereceived via the connection with the serving cell and an absence of asuitable neighbor cell. Responsive to detecting both the conditionindicative of a deterioration in the service received via the connectionwith the serving cell and the absence of a suitable neighbor cell, theprocessor initiates discovery of relay nodes.

In accordance with another aspect, a computer program product forenabling switching between modes of operation for a device in a cellularnetwork is provided. The computer program product includes anon-transitory computer readable storage medium having computer readableprogram code embodied therewith. The computer readable program codecontains instructions for providing a service to the device anddetermining, while providing the service to the device, that the deviceis approaching an edge of coverage. The computer readable program codecontains further instructions for, responsive to determining that thedevice is approaching the edge of coverage, sending a relay discoverycommand to the device; receiving a measurement report from the deviceindicating discovered nodes capable of acting as relay nodes; selectinga suitable node to act as a relay node; and instructing the device toinitiate a mechanism to switch to receiving the service via the relaynode.

According to another aspect, a serving cell is provided for enablingswitching between modes of operation for a device operating in acellular network. The serving cell includes a processor and acommunication subsystem. The communication subsystem is communicativelycoupled with the processor. The communication subsystem provides aservice to the device, and responsive to the processor determining thatthe device is approaching an edge of coverage: sends a relay discoverycommand to the device and receives a measurement report from the deviceindicating discovered nodes capable of acting as relay nodes. While thecommunication subsystem is providing the service to the device, theprocessor determines that the device is approaching an edge of coverage.Responsive to receiving the measurement report from the device, theprocessor selects a suitable node to act as a relay node and instructsthe device to initiate a mechanism to switch to receiving the servicevia the relay node.

It should be noted that although the examples provided herein relate to3GPP and LTE, the proposed solutions are not limited to those examplesand may be applicable to other systems or Radio Access Technologies,such as (but not limited to) 3GPP GSM EDGE Radio Access Network (3GPPGERAN) or 3GPP UMTS Terrestrial Radio Access Network (3GPP UTRAN), IEEE802.11, CDMA2000, etc.

In addition, the names used for code-points, information elements andmessages are only examples, and other names may be used. Furthermore,although the description of the solution might refer to a specificapplication (e.g. MCPTT), the solutions presented here are not limitedin applicability to any particular application. Additionally, the terms“UNR,” “relay” and “relay node” are used interchangeably herein.

Referring now to FIG. 1, User-to-Network Relays (UNRs) may be used forextending network coverage for Mission Critical Push-To-Talk (MCPTT).UE-13, UE-4, and UE-5 are acting as UNRs 102 a, 102 b, 102 c (referencedgenerally or collectively as UNR 102). A UNR 102 communicates with eNB108 of the LTE network 100 through the LTE-Uu (Uu) radio interface andis able to connect a remote User Equipment (UE) 104 a-104 j (referencedgenerally or collectively as UE 104) that is outside radio networkcoverage to the LTE network 100. The UNR 102 then relays downlink(network-to-UE) and uplink (UE-to-network) transmissions over the ProSeUE-to-UE Sidelink radio interface (PC5).

As illustrated, the network may use multicast (e.g., MultimediaBroadcast Multicast Service (MBMS)) or unicast (Evolved Packet System(EPS) bearers) transmission types. In this example, the GroupCommunication Service Application Server (GCS AS) 106 is the MCPTTapplication server. Multicast service is provided as enhanced MBMS(eMBMS) via Broadcast-Multicast Service Center/MBMS (BM-SC/MBMS) gateway112. eMBMS transmission links between the GAS AS 106 and the LTE UE 104,referred to as the LTE-Uu (Uu) are denoted as thick dashed lines.Unicast transmission links are provided via the Packet Data Network(PDN) gateway 110 and are denoted as thick solid lines, GCS AS 106communicates with BM-SC/MBMS gateway 112 and PDN gateway 110 via anInternet Provider (IP) Network 111.

The network 100 can directly provide the MCPTT service to MCPTT UEs thatare within radio coverage of an eNB 108 a, 108 b, 108 c (referencedgenerally or collectively as eNB 108) in Network Mode Operation (NMO)mode. In FIG. 1, UE-2 104 c, UE-3 104 d and UE-6 104 j are operating inNMO. UE-2 104 c, UE-3 104 d and UE-4 (UNR 102 b) are within broadcastrange 114 of eNB 108 b.

On the other hand, out of coverage UEs 104 may receive the MCPTT servicevia UNRs 102 in a mode referred to as Network Mode Operation via Relay(NMO-R). In FIG. 1, UE-14 104 a and UE-15 104 b are operating in NMO-Rthrough UE-13 (UNR 102 a), UE-7 104 e and UE-8 104 f are operating inNMO-R through UE-4 (UNR 102 b), while UE-9 104 g, UE-10 104 h and UE-11104 i are operating in NMO-R through UE-5 (UNR 102 c). In FIG. 1, UNRdownlinks relaying over PC5 are denoted as thin solid lines.

In addition, UE-14 104 a is in use by the current talker of the MCPTTgroup, and UE-13 (UNR 102 a) is the UNR in charge of transferringtalker's voice to the eNB 108 a and eventually to the GCS/MCPTTapplication server 106. UNR uplink relaying over PC5 is denoted as athin dotted line.

It should be noted that both end-user Public Safety (e.g., MCPTT)service provision and UE-to-Network relaying functions may be activatedon a single UE. However, for the sake of clarity, these functions arefurther considered as independent functionalities. It should also benoted that both the application media stream (e.g., voice frames) andthe corresponding signaling (e.g., Session Initiation Protocol (SIP)signaling messages) are relayed to/from out-of-coverage UEs (this infersthat a listening-only UE may use uplink transmission in certain phasesof a group call).

NMO to NMO-R Switch

For a UE receiving MCPTT service from the network in NMO, transitioningto NMO-R largely comprises two distinct phases:

a) discovering suitable UNR; and

b) executing a procedure to move the NMO bearers over Uu to NMO-Rbearers over PC5.

Referring now to FIG. 2, a flowchart 200 is provided which illustratesan example process for switching a UE 104 from operating in NMO mode tooperating in NMO-R mode. It should be noted, in the followingdescriptions, that the term “network” is used to indicate theinfrastructure element to which the device (either UE 104 or UNR 102depending on the context) is receiving the service from. Typically, thisinfrastructure element will be an LTE eNB 108.

Beginning at block S202, the UE 104, operating NMO mode, upon satisfying(at block S204) the triggering conditions for discovering UNRs 102,initiates (at block S206) UNR discovery. UNR discovery may be triggeredby either the UE 104 or the network, and methods of triggering discoveryby both the UE 104 and the network are discussed in further detailbelow. The UE 104 starts to attempt UNR discovery based on thedeterioration of serving cell quality/signal strength and the absence ofsuitable neighbor (i.e. non-UNR) cells (i.e. edge of radio networkcoverage). The network may trigger UNR discovery based on, for example,failed handover attempt (target cell overloaded, etc.). If whilstdiscovering a UNR, the quality of the service received via the networkimproves or if a suitable target neighbor cell is found (thus providinga way for the UE to continue the NMO), the UE may stop the UNR discoveryprocedures and stay in NMO (i.e. return to block S202). Upon satisfying(at block S208) the discovery and selection of a suitable UNR 102, theUE 104 then initiates (at block S210) mechanisms to switch to NMO-R atan appropriate time.

When the UE 104 is in NMO, it is receiving service via the network. Theapplication (e.g., the MCPTT application) should be oblivious to anychanges in the lower layer when the UE 104 switches to NMO-R. The PDCP,RLC, MAC and PHY layers in the LTE stack however need to be reconfiguredinto NMO-R mode of operation upon moving into NMO-R. The association ofvarious peer protocol layers in NMO and NMO-R are as shown in FIG. 3.

Triggering Conditions for Initiating Discovery of UNR

The detection of trigger conditions for switching from NMO to NMO-Rswitch, in turn, initiates the discovery of suitable UNRs 102. Anexample sequence may be measuring serving cell, measuring neighbor cell(NC), determining that the serving cell is low and there is no suitableNC, looking for UNRs 102 (i.e. performing discovery), and eventuallyswitching to NMO-R upon finding a suitable UNR 102. The UE 104 mayindicate to the network its preference to the network (or that certaincriteria are met) to switch to NMO-R, with or without identifying acandidate UNR 102 during this process. In RRC_CONNECTED mode, thispreference may indicate a request for the network to terminate the RRCconnection.

Certain devices having more than one transceiver may be capable ofperforming a “Make-Before-Break” (MBB) handover, which is discussed ingreater detail below. For both MBB-capable and non-MBB-capable devices,discovery may be initiated prior to RRC Connection Release in theserving cell.

A UE 104 may discover one or more relays 102 supporting the MCPTTservice (i.e. phase a) of NMO to NMO-R transition, mentioned above) theUE 104 is interested in to be able to switch to NMO-R operation (i.e.phase b of NMO to NMO-R transition, mentioned above). However, searchingfor relays 102 in the vicinity of UE 104 incurs additional powerconsumption at the UE 104. Performing discovery whilst being in NMO mayalso result in service interruption or degradation to the servicesreceived over the network depending on the UE 104 capabilities.

Hence, a UE 104 in RRC_IDLE or RRC_CONNECTED with good radio conditionsand using the MCPTT service in NMO with a satisfactory quality ofservice may not trigger UNR discovery. In principle, if the UE 104 findsa suitable neighbor cell when the serving cell quality degrades, thenthe UE 104 follows the normal procedures and reports target cellmeasurements to the eNB 108 (i.e. using measurement report) and dependson the eNB 108 for potential service continuity (e.g., handover (HO), asis the case currently in RRC_CONNECTED mode).

However, if the eNB 108, upon receiving the measurement report, makes adecision that handover is not suitable and instead NMO-R might benecessary (e.g., due to a high load in the reported neighbor cell,reported quality being not good enough, etc.), the eNB 108 may triggerthe UE 104 to initiate discovery of the relays 102 at the UE 104.

Additionally, a UE 104 in NMO may autonomously initiate UNR discoveryupon determining certain conditions calling for an imminent need fortransition to NMO-R. In such a case, the UE 104 shall initiate andcomplete the UNR discovery before the UE 104 is abruptly disconnectedfrom the network (e.g., by experiencing a Radio Link Failure). Thetriggering conditions for beginning the search for UNRs 102 may includeone or more of:

Detection of a condition indicating degrading network service; and

Detection of “Edge of coverage” condition.

One example of a condition indicating degrading network service is thedetection of radio link degradation on the Uu interface. Thisdegradation may include degradation of serving cell quality (e.g., RadioSignal Receive Quality (RSRQ) or Channel Quality Indicator (CQI)) belowa predetermined threshold, Such a predetermined threshold may besignaled to the UE 104 via RRC signaling or may be preconfigured in theUE (e.g., specified in the standards, configured in the UniversalIntegrated Circuit Card (UICC), etc.).

Anticipation of an imminent Radio Link Failure (RLF) is another exampleof a condition indicating degrading network service. Radio linkmonitoring is used to detect the quality of radio link between the eNB108 and the UE 104. The RLF procedure is used to trigger procedures thatthe UE 104 shall initiate upon detecting deterioration of the radio linkbetween the eNB 108 and the UE 104. Two phases govern the behaviorassociated to RLF. The first phase is started upon radio problemdetection (i.e. upon detecting a predetermined number of out-of-syncindications from physical layer) and leads to RLF detection. The UE 104continues to be in RRC_CONNECTED state and is based on timer or other(e.g., counting) criteria (T₁). The timer is referred to as T310 in 3GPPTS 36.331. The second phase is started upon RLF detection (i.e.subsequent to first phase) or handover failure and is also timer based(T₂) (i.e. a timer (referred to as T311 in 3GPP TS 36.331) is startedupon detecting the RLF. During phase two, the UE 104 initiates areestablishment procedure and attempts to reconnect to an eNB 108. Uponexpiry of the timer (T311), the UE 104 enters RRC_IDLE. Anticipation ofRLF may include one or more of a timer indicative of imminent radio linkfailure (such as T310 or T312) is running or a predetermined number of“out-of-sync” indications have been received. The predetermined numberof out-of-sync indications may be indicated to the UE 104 via RRCsignaling or may be preconfigured in the UE 104 (e.g., specified in thestandards, configured in the UICC, etc.).

Another example of a condition indicating degrading network service isservice quality degradation. The application or an underlying protocolsuch as Packet Data Convergence Protocol (PDCP) or Radio Link Control(RLC) detects that the quality of the received service has degradedbelow a predetermined threshold. For instance, this detection mayinclude detection of a predetermined number or percentage ofmissed/un-decoded voice frames, user data frames or IP packetspertaining to a media. This detection may also include determinationthat other key parameters, such as the residual bit error rate on theapplication packets, has exceeded a predetermined threshold, etc. Thesepredetermined numbers and thresholds may either be signaled to the UE104 via RRC signaling or they may be preconfigured in the UE 104 (e.g.specified in the standards, configured in the UICC, etc.).

Yet another example of a condition indicating degrading network serviceis the service becoming unavailable. In other words, the serving celldoes not provide the service (e.g., the MCPTT session or the eMBMSsession) the UE 104 is interested in (e.g., due to temporary lack ofresources).

An example of detection of “Edge of coverage” condition may includedetection of one or more of the above conditions related to degradingnetwork service in the serving cell whilst determining that there is nosuitable neighbor cell providing the service by which the UE 104 isinterested in. Edge of coverage may be detected based on the neighborcell measurements and also via the system information of the neighborcells to identify if the service is supported, for example, by readingthe System Information Block (SIB) 13 to see if the related service(e.g., MCPTT service or the eMBMS session, etc.) is available.

When a UE 104 is approaching the edge of coverage, none of the detectedcells including the serving cells and neighbor cells on the measuredfrequencies, would look good (i.e. there is no suitable cell as definedin 3GPP TS 36.304). For example, the received power of those cells maybe less than a threshold. If that is the case and the UE 104 has nottriggered any events for handover (for example, Event A3 as defined in3GPP TS 36.331), then NMO-R may be appropriate.

According to 3GPP TS 36.331, Event A2 is triggered if the servingfrequency signal becomes worse than a threshold. However, if themeasurement report does not contain any neighbor cell measurement, itmay be indicative of the Edge of coverage condition. Moreover, there isno event for reporting that non-serving cells become worse than athreshold. A new event, e.g., A7, may be defined and will be triggeredwhen a non-serving frequency becomes worse than a threshold. When thenetwork receives both A2 and A7 triggers, the network may assume thatthe UE 104 is approaching the edge of the coverage.

Note that some or all of the above triggering conditions may be detectedby either the UE 104 or the eNB 108 or both. Upon satisfying thetriggering conditions for initiating discovery of UNR 102, the UE 104shall proceed to phase b) of the procedure for transitioning to NMO-R(i.e. the UE 104 shall initiate the UNR discovery procedure).

UE Triggered Mechanisms to Switch to NMO-R

Once the UE 104 initiates NMO to NMO-R switching mechanism (i.e. thephase b) of the transitioning to NMO-R), depending on the RRC state ofthe UE 104 in NMO, the UE 104 may need to execute different mechanismsto eventually complete the NMO to NMO-R switch. Details of the switchingmechanisms depending on the UE's RRC state are also discussed in furtherdetail below.

UE in RRC_CONNECTED State

Two approaches for switching from NMO to NMO-R for a UE 104 inRRC_CONNECTED state are disclosed: Break-Before-Make (BBM) andMake-Before-Break (MBB). In BBM, the MCPTT service is re-establishedthrough the UNR 102 over the PC5 interface after the RRC connection hasbeen released and accesses the MCPTT services through the eNB 108 overthe Uu interface is interrupted. Using MBB, the MCPTT service is handedover from the eNB/Uu path to the relay/PC5 path before the RRCconnection is released and related access to MCPTT services isuninterrupted. Depending on the UE 104 capability (i.e. on whether theUE 104 supports simultaneously PC5 bearers and Uu bearers) andcriticality of the MCPTT service, a choice between MBB and BBMprocedures is made. This decision can be made by the UE 104 and signaledto the network, or the decision can be made at the network (e.g., basedon the information provided by the UE 104).

“Break-Before-Make” (BBM):

Using this first approach, depicted by event flow diagram 400 in FIG. 4,the UE 104 is operating in NMO in the RRC_CONNECTED state. MCPTT service(i.e. data and control) is provided directly from the serving eNB 108 tothe UE 104 over the Uu radio link. At step S401, the UE 104 detects thata condition to initiate UNR discovery exists.

The UE 104, at step S402, performs the ProSe Direct Discovery of UNRs102 in communication range able to provide connectivity for the servicethe UE 104 is interested in and selects an appropriate relay 102. ProSeDirect Discovery consists of a set of procedures used by ProSe enabledUEs or ProSe relays supporting Direct Discovery to detect and identifyother ProSe-enabled UE(s) or ProSe relay(s) in their proximity, usingE-UTRA direct radio signals via PC5. It should be noted that EPC-levelDiscovery (by which the Enhanced Packet Core determines the proximity ofthe UEs and informs them of their respective proximity) should bedistinguished from ProSe Direct Discovery. 3GPP TS 23.303 specifies twodiscovery models, Model A and Model B.

Model A (“I am here”) defines two roles for the ProSe-enabled UEs/ProSerelays that are participating in ProSe Direct Discovery: the AnnouncingUE announces certain information that could be used by UEs in proximitythat have permission to discover and the Monitoring UE monitors certaininformation of interest in proximity of announcing UEs. In this model,the announcing UE broadcasts discovery messages at pre-defined discoveryintervals and the monitoring UEs that are interested in these messagesread and process these messages.

Model B (“who is there?”/“are you there?”) defines two different rolesfor the ProSe-enabled UEs/ProSe relays that are participating in ProSeDirect Discovery: the Discoverer UE transmits a request containingcertain information about what it is interested to discover and theDiscoveree UE receives the request message and can respond with someinformation related to the discoverer's request.

The following information may be used for ProSe UNR discovery andselection:

-   -   Message type identifier (e.g., identifying Model A or Model B        discovery);    -   ProSe Relay (UE) ID: link layer identifier that is used for        direct communication and is associated with a PDN connection the        ProSe UNR has established;    -   PLMN ID: identifies the Public Land Mobile Network (PLMN) to        which radio frequencies used on the link to the Remote UE        belong. If these radio frequencies are shared between multiple        PLMNs, or not allocated to any PLMN, then the choice of PLMN ID        is configured by the Home PLMN (HPLMN);    -   ProSe Application Relay Code: parameter identifying connectivity        the ProSe UNR provides (e.g., including Access Point Name (APN)        information);    -   Whether the discovered UE can act as a relay (i.e. whether a UE        that has been discovered can act as an UNR); and    -   Status/maintenance flags (e.g., indicating whether the relay is        temporarily without connectivity or battery running low so the        Remote UEs can seek/reselect another Relay).

Returning now to FIG. 4, in order to enable the exit from NMO, the UE104 sends, at step S403, a NMO-R Preferred (i.e. a relay modepreference) indication to the network. This indication may, implicitlyor explicitly, express a request for releasing the RRC connection. Forexample, the release of the RRC connection may be performed for a devicenot supporting concurrent transmission on Uu and PC5, hence unable toswitch to NMO-R in RRC_CONNECTED, while this is not performed for adevice capable of simultaneous transmission on Uu and PC5. On receipt ofthe NMO-R Preferred indication, at step S404, the network may determinethat the RRC connection should be released.

If the network determines, at step S404, that the RRC connection shouldbe released, the network sends, at step S405, a RRC Connection Releasemessage to the UE 104. A new release cause value is set in the RRCConnection Release message in order to indicate to the UE to not triggerservice request procedure and keep the existing EPS bearers. Optionally,the network may also include, in the RRC Connection Release message,identities of any other target relays 102 that the network may deemappropriate. The network will know the approximate location of the UE104 and may for instance be aware of UNRs 102 operating in the proximityof the UE 104 and indicate the UNRs' 102 identities for the UE 104 todiscover. The UE 104 may use these UNR identities to perform asubsequent discovery step to find if a more suitable UNR 102 may befound. These identities may be included in the RRC Release message orsent separately from the release message.

Upon releasing the RRC connection, the eNB 108 may also initiate S1bearer release for the UE 104. Alternatively, the eNB 108 may keep thecorresponding S1 bearers and redirect the user plane traffic to the UNR102. The network may choose to release the UE context at this pointalthough the UE 104 keeps the context of the PDN connection locally. Thesteps described below are independent of how the traffic is rerouted tothe UNR 102 and whether or not the network releases the UE context. Inother words, the subsequent steps are independent of whether the UE 104may be considered as attached or detached as far as network isconcerned. If the network does not release the RRC connection, the UE104 remains in NMO and does not initiate the switch to NMO-R,until/unless a RLF is experienced and the UE 104 loses network Uuconnectivity.

The UE 104 performs, at step S406, procedures described in FIGS. 5and/or 7 to switch to NMO-R, depending upon whether or not the UE 104 iscurrently in an established MCPTT session.

During one-to-one connection establishment, the UE 104 may request theUNR 102 to relay the existing PDN connection(s), as shown in FIG. 5. TheUE 104 establishes a one-to-one connection with a UNR 102 capable ofrelaying the PDN connection(s) for the services to be carried over PC5interface and requests, at step S501, the UNR 102 to relay this (these)PDN connection(s). This request can be complemented by relevant UEcontext information to the UNR 102. The relevant UE context informationmay indicate the PDN connection(s) and the related APNs. The UE contextmay also include the Quality of Service (QoS) and other parametersrelated to the EPS bearers used by the UE 104 while in NMO.

The UNR 102 requests, at step S502, bearer resource modification orallocation from the network based on the information received from theUE in step S501 by transmitting Bearer Resource Allocation Request orBearer Resource Modification Request to the network. In return thenetwork may modify the already established EPS bearers between UNR 102and eNB 108 (e.g., bearers serving UNR's 102 own communication needs orbearers for relaying transmissions for other out of coverage UEs 104) orallocate new dedicated EPS bearers. This step ensures that the Uu linkbetween the UNR 102 and the eNB 108 can efficiently serve the out ofcoverage UE 104.

The UNR 102 assigns, at step S503, logical channel identities of PC5bearers corresponding to the EPS bearers to be relayed. The UNR 102maintains the following information per EPS bearer to be relayed forrelaying operation over PC5.

-   -   a. L2 source address of the UE 104;    -   b. IP address of the UE 104 assigned by the UNR 102;    -   c. Identity of the EPS bearer(s) that the UE 104 requested;    -   d. Identity of the UNR's EPS bearer which is now associated with        the EPS bearer identity the UE 104 requested (i.e. transporting        the corresponding data); and    -   e. Sidelink logical channel identity assigned to the EPS bearer        in c and d.

At step S504, the UNR 102 responds to the UE 104 with the sidelinklogical channel identities corresponding to EPS bearers. The UE 104establishes the PC5 bearers and associates the logical channelidentities with the corresponding EPS bearers.

Further variants of the BBM approach presented above can be considered,such as, upon reception of the NMO-R Preferred indication in step S403,the network may elect to send a newly defined indication Switch to NMO-RDeferred instead of the RCC Connection Release in step S405, as a resultof which the UE 104 remains in NMO and does not initiate the step S406,until/unless a RLF is experienced.

As a further alternative, instead of sending a Switch to NMO-R Deferredindication, the network may send the ProSe configuration applicable inthe cell to the UE 104 to enable NMO-R operation. This option isapplicable when the ProSe frequency belongs to the serving cell. Thisoption assumes that the UNR 102 is also using the same ProSeconfiguration (e.g., since the UNR 102 is connected to the same eNB 108or to an eNB 108 whose ProSe configuration is known to the serving eNB).

Additionally, the UE 104 starts a timer at the sending of the NMO-RPreferred indication to the network at step S403. If the timer elapsesbefore the UE 104 receives a RRC Connection Release or a Switch to NMO-RDeferred (variant mentioned above), the UE 104 initiates step S406 ifcapabilities allow.

Sending the NMO-R Preferred indication to the network may be leftoptional. By default, if the UE 104 switches to NMO-R withouttransmitting this indication, the eNB 108 may send the UE 104 toRRC_IDLE due to inactivity.

“Make-Before-Break” (MBB):

In the MBB approach, the UE 104 performs discovery of suitable relay(s),may interact with the network before proceeding to NMO-R establishment,then switches to NMO-R. This approach minimizes the service interruptiontime incurred during the establishment of NMO-R as the UE 104 supportsdiscovery of UNRs 102 and establishment of NMO-R whilst in RRC_CONNECTEDstate.

Referring to FIG. 6, the UE 104 detects, at step S601, a condition toinitiate UNR discovery as described above (i.e. a trigger). The UE 104performs, at step S602, the discovery of UNRs 102 in communication rangeable to provide connectivity for the service the UE 104 is interested inand selects an appropriate relay, as described above. The UE 104 maysend, at step S603, a NMO-R Preferred indication to the network to makethe network aware of the UE's intention to switch to NMO-R in a shortterm and to get the authorization from the network to perform theswitch.

The network may answer, at step S604, the NMO-R Preferred indicationwith a NMO-R Proceed indication to permit the UE 104 to proceed toswitch immediately. In some implementations, the NMO-R Proceedindication may include the ProSe configuration of the cell (this optionis applicable when the ProSe frequency is owned by the serving cellthereby enabling the UE 104 to adopt the signaled ProSe configurationfor NMO-R operation). As an example, upon receiving NMO-R preferredindication, the network may include ProSe configuration parametersallowing the UE 104 to autonomously select resources from resource poolsto transmit Sidelink Control and data or discovery messages (i.e. UEautonomous resource selection, also referred to as Mode 2 DirectCommunication or Type 1 Discovery), thereby enabling the UE 104 to usethe resources when out of coverage. In case the network does not sendsuch indication, the UE 104 remains in NMO (which would result in aBreak-Before-Make scenario). In another option, the network may includethe details of target UNRs 102 (i.e. the ProSe layer 2 IDs of the targetrelays 102) in the NMO-R proceed indication. The target UNR informationmay help the UE 104 in performing a further discovery step to discover amore suitable UNR 102 if appropriate. It is noted that steps S603 and/orS604 may be optional.

During the one-to-one connection establishment with the UNR 102, at stepS605, the UE also performs the steps described in FIG. 5. The UE 104then operates in NMO-R while still in RRC_CONNECTED state.

At step S606, the UE 104 indicates that it has completed the switch toNMO-R to the network by sending an NMO-R Entered indication. On receiptof the NMO-R Entered indication, the network may release, at step S607,the RRC connection and the UE 104 enters RRC_IDLE. A new release causevalue may be set in the RRC Connection Release message in order toindicate to not trigger service request procedure and keep the existingEPS bearers. When the UE 104 establishes corresponding PC5 bearers,these PC5 bearers are associated with the EPS bearers. If the UE 104does not inform the network of the switch, or if the network does notrelease the RRC connection, the UE 104 remains in RRC_CONNECTEDuntil/unless a RLF is experienced and the UE 104 loses network Uuconnectivity.

Further variants of the Make-Before-Break approach presented above canbe considered, such as, upon reception of the NMO-R preferred indicationat step S603, the network may elect to send a newly defined indicationSwitch to NMO-R Deferred, as a result of which the UE 104 remains in NMOand does not initiate the switch to NMO-R, until/unless a RLF isexperienced. Additionally, the UE 104 may start a timer at the sendingof the NMO-R preferred indication to the network, at step S603. If thetimer elapses before the UE 104 receives a NMO-R proceed or a Switch toNMO-R deferred, the UE 104 initiates the switch to NMO-R.

Choice Between Break-Before-Make and Make-Before-Break

UE capabilities may be considered in the choice between BBM and MBB. AUE 104 that can support NMO-R whilst in RRC_CONNECTED state can adopt aMake-Before-Break approach (i.e. according to FIG. 6) whereas a UE 104that is not capable of supporting NMO-R in RRC_CONNECTED state willemploy the Break-Before-Make approach (according to FIG. 4).

For UEs 104 that can support both MBB and BBM, the choice betweenBreak-Before-Make and Make-Before-Break approaches may further depend onthe criticality of the service in use (e.g., on the priority of theMCPTT group call in which the user is involved), whether the UE 104 “hasthe floor” and is engaged in uplink transmission, the ProSeconfiguration, or other QoS related criteria. Typically,Make-Before-Break should be used in case of high priority ordelay-sensitive communications, or if the MCPTT user is the currenttalker. Although it is assumed that discovery of UNRs 104 may beperformed in parallel to NMO, discovery may incur a power penalty ashighlighted above. In case of choosing a BBM strategy, the UE 104 candefer discovery until the UE 104 effectively loses network coverage(i.e. the NMO leg is broken). This further minimizes the number ofdiscovery attempts and may be appropriate for delay tolerant bearerswhere BBM is selected.

If the ProSe resources to enable NMO-R are available for the UE 104 onlyin one of the RRC states (e.g., only in RRC_CONNECTED state—i.e.operating scheduled resource allocation only) then the eNB 108 may keepthe UE 104 in RRC_CONNECTED state. On the other hand, if the ProSeresources are also available for the UE 104 in RRC_IDLE state (i.e. UEautonomous resource selection is applicable) then the eNB 108 may chooseto send the UE 104 to RRC_IDLE state depending on other criteria asmentioned above. Note that availability of pre-allocated ProSe resourcesin RRC_IDLE state may be helpful for the UE 104 to be able to receivethe service when the UE 104 is totally out of coverage.

The above choice between the two approaches may be made at the UE 104 orat the network or may be a cooperative decision between the UE 104 andthe network based on some interaction between them. For instance, the UE104 may select a preference for one of the above approaches (i.e.Make-Before-Break or Break-Before-Make) and may indicate this preferenceto the network using the NMO-R preferred message. The network may thenconsider the preference/choice indicated by the UE 104 along with othercriteria for deciding between the approaches as mentioned above. Upondeciding on an approach, the chosen approach is then executed as perFIG. 4 or FIG. 6. Specifically, the network, upon receiving anindication indicating preference for NMO-R, responds by sending a RRCConnection Release message to a UE 104 only supporting BBM and may keepthe UE 104 in RRC_CONNECTED if the UE 104 supports MBB. If the UE 104supports MBB, the network may further decide to still send a RRCConnection Release message to the UE 104 if deemed appropriate based onthe criticality of the active NMO bearers (e.g., in case of delaytolerant NMO bearers, the network may choose to release the RRCconnection—this option will be useful, for instance, when the network iscongested and releasing the UE 104 earlier would help releasing thecongestion or reducing interference situation in the network, etc).

UE Preference Indication

Any of the following may be used for indicating UE preference to switchto NMO-R (i.e. NMO-R preferred or NMO-R Entered indications in abovefigures):

-   -   An RRC message defined to convey this information;    -   By indicating UE preference in ProSe related signalling (i.e.        within the ProSe UEInformation RRC indication);    -   As an example, a new cause code could be included in the        ProSeUEInformation indication to indicate to the network that        NMO-R is preferred;    -   By sending a detach request message. The UE 104 may also include        an indication indicating to the network that the detach request        is due to preference to switch to NMO-R. In this case, even        though the UE 104 sends a detach request message to the network,        the UE 104 still keeps the corresponding UE context and switches        the Uu bearers to the corresponding PC-5 bearers once the NMO-R        is activated. Hence, from the network perspective, the UE 104        may be considered as in “detached” state whilst the UE 104 may        store part or all of the UE context information. Alternatively,        the network may also keep the UE context. In other words,        consider the UE 104 to be in attached state and the network will        adopt this different behavior of retaining the UE context of a        UE sending “detach” message based on the cause code indicated        for the detach (i.e. a cause code indicating that UE 104 is        requesting detach to enter NMO-R mode);    -   By using a new MAC control element (MAC CE);    -   By using an indication for a power optimised configuration on        the network interface (e.g., by using UE assistance information        message); or    -   By including a new information element or indicator indicating        the preference for switch to NMO-R in any of the messages        mentioned above.

UE in RRC_IDLE State

A UE 104 in RRC_IDLE state may be receiving MCPTT service (e.g., viaeMBMS). In this case, the UE 104 may autonomously switch to NMO-R upondetecting suitable trigger conditions for such a switch as depicted inFIG. 7. The UE 104 in RRC_IDLE detects, at step S701, that a conditionto switch from NMO to NMO-R has been triggered as described above,(e.g., radio link degradation, service quality degradation, etc.) The UE104 performs, at step S702, the discovery of UNRs 102 in communicationrange able to provide connectivity for the service in which the UE 104is interested and selects an appropriate relay 102. The UE 104 performs,in step S703, the operations for the establishment of NMO-R according tothe processes described in FIG. 4.

Network Triggered/Assisted Switch to NMO-R

In this instance, the UE 104 is assumed to be in RRC_CONNECTED state inNMO. The network facilitates the UE performing a switch to NMO-R. In onescenario, the network knows the UE capabilities and also its coveragesituation (e.g., based on the measurement reports sent by the UE 104).As in the UE triggered switch to NMO-R, both Make-Before-Break andBreak-Before-Make approaches are feasible as well.

When the UE 104 is approaching to the edge of coverage (i.e. triggeringconditions satisfied as described above), the eNB 108 instructs the UE104 to start looking for a UNR 102 in proximity. This scenario isdepicted in FIG. 8, which shows the Make-Before-Break case in which theUE 104 is capable of supporting NMO-R whilst in RRC_CONNECTED.

The eNB 108 detects, at step S801, that triggering conditions forinitiating UNR discovery have been met as described above. The eNB 108sends, at step S802, a Relay discovery command message to the UE 104.Alternatively, this command may be an enhanced measurement configurationmessage. The UE 104 performs, at step S803 a UNR discovery procedure.The UE 104 may optionally obtain, via this procedure, the cell-relatedidentifiers (e.g., C-RNTI) of the UNRs 102 discovered. The cell-relatedidentifiers of the discovered UNRs 102 may be used by the eNB 108 infurther steps of the procedure. The UE 104 reports, at step S804,information about one or more UNRs 102 discovered to the eNB 108 (e.g.,the received signal power and quality measurements, the L2 sourceaddress, the battery level and the available processing power, the UNR'sserving cell identifier, etc.) This information may be included in ameasurement report, however, a new message may be defined, (e.g., aRelay discovery response message). The UE 104 may also provideadditional information such as its own location either included in theabove message or in addition to the above messages to facilitate the eNB108 to find and configure UNRs 102 in the geographical area where the UE104 is located.

The eNB 108, at step S805, selects one of the discovered relays 102 andinstructs the UE 104 to establish one-to-one Sidelink communication(i.e. over PC5) with the selected relay 102 if the UE 104 supportssimultaneous Sidelink and Uu communications with a NMO-R mode command.The indication may be conveyed in a RRC connection reconfigurationmessage. The UE 104 is aware of existing logical channels over Uu andtheir QoS parameters, e.g., logical channel priority and bit ratesserved by the eNB 108. The UE 104 may establish the same number ofSidelink logical channels over the PC5 interface with similar QoSparameters as for the logical channels used over the Uu interface.Alternatively the eNB 108 may instruct the UNR 102 to establish aone-to-one Sidelink communication with the UE 104. The eNB 108 is awareof the established logical channels over the Uu interface. Theinformation about the logical channels may be conveyed to the UNR 102 torequest the UE 104 to configure the same number of logical channels withsimilar QoS characteristics.

The UE 104, at step S806, establishes a one-to-one Sidelinkcommunication with the UNR 102 as described above. The UE 104 informs,at step S807, the eNB 108 that the UE 104 has successfully establishedthe Sidelink communication with a NMO-R Entered indication. Thisindication may be a RRC connection reconfiguration complete message. Thenetwork decides, at step S808, whether RRC connection needs to bemaintained. The network may, at step S809, send a RRC Connection Releaseto the UE 104 to instruct the UE 104 to enter RRC_IDLE state. Upontransition to RRC_IDLE the UE 104 switches from the logical channelsover Uu to PC5.

If the UE 104 is unable/not capable of supporting NMO-R in RRC_CONNECTEDstate, one alternative is to adopt Break-Before-Make strategy (otherreasons for a choice between Make-Before-Break and Break-Before-Make asdescribed previously are also applicable in making this decision). Inthis case, the UE 104 receives an RRC connection release message priorto the switch to NMO-R. This procedure is depicted in FIG. 9.

Steps S901 through S904 are substantially similar to steps S801 throughS804 in FIG. 8 as described above. In step S905, the eNB 108 selects oneUNR 102 and instructs the UE 104 to establish a one-to-one Sidelinkcommunication with the selected UNR 102. The instruction may beconveyed, in step S906, in RRC Connection Release if the UE 104 does notsupport simultaneous Uu and Sidelink communications. The UE 104establishes one-to-one Sidelink communication with the UNR 102 asdescribed above, starting NMO-R operation.

In order to facilitate the above scenarios, one or more new indicationsfrom the eNB 108 to the UE 104 may be defined. Specifically, to triggerdiscovery of relays 102 at the UE 104, an indication referred to asRelay discovery command (see e.g., FIG. 8, step S802) may be sent by theeNB 108 to the UE 104. The eNB 108 may configure one or more UEs inrelay mode prior to sending this message to the UE 104 if the eNB 108 isaware that there are no potential relays close to the UE 104. The UE 104starts relay discovery upon receiving this indication (see e.g., FIG. 8,step S803).

To trigger the UE 104 to switch to NMO-R, an indication referred to asNMO-R mode command may be sent by the eNB 108 to the UE 104 (see e.g.,FIG. 8, step S805). The UE 104 establishes NMO-R upon receiving thisindication. This command can include the identity of the relay 102 withwhich the UE 104 should associate. Any UNR identity such as the C-RNTIof the relay UE 102 or the ProSe UE ID (i.e. the source Layer-2 ID) ofthe UNR 102 may be used for this purpose.

The UE 104 may confirm the completion of an NMO-R switch to the eNB. 108by sending an indication referred to as NMO-R Entered (see e.g., FIG. 8,step S807). The eNB 108 may initiate mechanisms to consolidate andpotentially release the RRC connection of the UE 104 (e.g., when noother service configured to use Uu interface is active) upon receivingthis indication (see e.g., FIG. 8, step S808).

Any of the above indications may be included in an existing or a new RRCmessage, or may be conveyed via a new MAC Control Element.

Further, the measurement report message, defined by 3GPP TS 36.331, maybe enhanced to also indicate the discovered relays (see e.g., FIG. 8,step S804). The Source Layer-2 ID (ProSe UE ID), defined by 3GPP TS23.303, or the C-RNTI of the UNR 102 can be included in the measurementreport message for this purpose. The network may select one of thereported UNRs 102 as the preferred candidate for connecting the UE 104and can indicate this in the NMO-R mode command (see e.g., FIG. 8, stepS805). Alternatively, the network may indicate a subset of relays, orotherwise, a ranked list of relays, in the NMO-R mode command. Thenetwork may prioritize the relays 102 within its own coverage over therelays 102 that are not in its coverage.

As a further option, the eNB 108 may interrogate one or more UNRs 102about their capacity to support an additional incoming UE 104. Thisinformation may be helpful for load balancing purposes between the UNRs102. Communication between the eNB 108 and UNRs 102 may be as shown inFIG. 10.

The Incoming UE request message in step S1001 may include the UEidentifier of the potential incoming UE 104. If there is more than onepossible UNR 102 under the eNB control, the eNB 108 may select one UNR102 or prepare more than one UNR 102 for the incoming UE 104 byproviding the potential UNRs 102 with the UE identifier. This UEidentifier can be the ProSe UE ID of the UE 104 or any other identity bywhich the UNR 102 can identify the incoming UE 104 on the PC5 link. TheUE ID of the incoming UE 104 may be used by the relay 102 to establishthe Sidelink connection. As a response to this message, the UNR 102 maysend the Incoming UE response message, at step S1002. This message caninclude parameters that the eNB 108 can use to select a UNR 102 amongmultiple relays candidates. Examples of these parameters are supportedapplications/application IDs, battery status, mobility status,position/geographical location in the cell, load (e.g., number of out ofcoverage UEs currently associated with the UNR 102, or relative loadpercentage), number of MCPTT groups which the relay forwards, anexplicit indication to reject the additional incoming UE 104, etc.

Based on these parameters, the eNB 108 can select an appropriate UNR 102and include the identity (ProSe UE ID) of the selected relay 102 in theNMO-R mode command transmitted to the UE 104 (see e.g., FIG. 8, stepS805). Alternatively the Incoming UE request may be utilized for the eNB108 to instruct the UNR 102 to establish a one-to-one connection withthe UE 104. In this case, the request may include the UE's L2 sourceaddress and the logical channels to be established over Sidelink.

NMO-R to NMO Switch

A UE 104 in NMO-R mode of operation may move into network coverage whereNMO mode of operation is potentially available. NMO can be availablewhen the PDN connectivity to the service can be provided by the network.NMO availability can be determined by the UE 104 based on the systeminformation of the network (i.e. availability of MBMS sessions that theUE 104 is interested in, etc).

In this case, two approaches are disclosed:

-   -   1) The UE switches to NMO upon detecting network coverage        supporting NMO.    -   2) The UE stays in NMO-R until a switch to NMO is deemed        necessary based on triggering criteria such as operation in        NMO-R deteriorating, etc.

The UE 104 may be preconfigured to choose between these approaches(e.g., configured in the UICC or via explicit signaling from thenetwork). Alternatively, one specific behavior may be enforced by thestandards. The behavior may also be based on the capability of the UE104 (i.e. whether or not the UE 104 can support NMO-R whilst inRRC_CONNECTED).

UE Always Switches to NMO Upon Finding Coverage

In this instance, an out of coverage UE 104 operating in NMO-R alwaysswitches to NMO upon moving into network coverage. Thus, while out ofcoverage, the UE 104 will be performing cell search (as per standardizedcell selection/reselection algorithms) until a suitable cell is found,and selects a suitable cell when available (see 3GPP TS 36.304). Uponselecting the cell, the UE 104 may enter connected mode and update itsregistration with the MCPTT server 106 via the network. If the UE 104supports MBB, the UE 104 may initiate establishment of NMO and uponsuccessful registration with the MCPTT server 106 and resuming access tothe service, the UE 104 may detach from the UNR 102 and switch to NMO.This process is depicted in FIG. 11.

The UE 104 operating in NMO-R through the UNR 102 enters networkcoverage and selects a suitable cell, at step S1101. The UE 104establishes, at step S1102 an RRC connection in order to get MCPTTservice in the serving cell. The UE 104 accesses, at step S1103, theMCPTT service using IMS/SIP procedures after mutual authentication andestablishment of secure association (SA-R) between the UE 104 and theMCPTT server 106. If needed, (i.e. not prevented by the MCPTT server106), the UE 104 may have to suppress duplicate information that couldbe temporarily received from the relay 102 and from the network. The UE104 sends, at step S1104, a Sidelink Disconnect indication to the UNR102 to stop the relay transferring MCPTT information for this UE 104.The UNR 102 stops transmissions directed towards the UE 104, at stepS1105. The UNR 102 may answer the Sidelink Disconnect indication by aSidelink Disconnect Ack, at step S1106. If there are no more UEs 104employing relaying operation by the UNR 102, then the UNR 102 may ceaseits relaying activity, at step S1107, and send a UNR Mode Stopindication to the serving eNB 108. It should be noted that the servingeNB of the UNR 102 may or may not be the same one as the serving eNB forthe UE 104. As an example, the UNR Mode Stop indication may be includedin an RRC message, such as ProSe Interest Indication, indicating thatthe UNR 102 is no longer interested in ProSe.

The procedure discussed with respect to FIG. 11 works when the UE 104can support NMO-R while in RRC_CONNECTED state. However, if the UE 104is not capable of this, the UE 104 may switch to NMO after disconnectingfrom the UNR 102. This choice of using Break-Before-Make orMake-Before-Break may involve other considerations as mentioned above.

The “Break-Before-Make” option is depicted in FIG. 12. The UE 104operating in NMO-R through the UNR 102 enters network coverage, at stepS1201 and selects a suitable cell. The UE 104 sends, at step S1202, aSidelink Disconnect indication to the UNR 102 to stop the relaytransferring MCPTT information for this UE 104. The UNR 102 stopstransmissions directed towards the UE 104, at step S1203. The UNR 102may answer the Sidelink Disconnect indication, at step S1204 by aSidelink Disconnect Ack.

The UE 104 establishes, at step S1205, an RRC connection in order to getMCPTT service in the serving cell. This procedure involvesestablishment, at step S1205 a, of the EPS bearers corresponding to theservices that the UE 104 is receiving over the PC-5 link. A Non-AccessStratum (NAS) layer in UE 104 triggers, at step S1205 b, the servicerequest procedure to establish the needed EPS bearers corresponding tothe bearers that the UE is receiving the service over when in NMO-R. Theservice request message may be forwarded by the eNB 108 to the MME toconfigure appropriate EPS bearers for the UE 104. The eNB 108 respondsby sending an RRC Configuration, at step S1205 c, to the UE 104 and thisRRC configuration includes the configuration of the EPS bearers and DRBsto serve the UE 104 in NMO. The UE 104, at step S1205 d, associates theapplication data flows corresponding to the PC-5 bearers to theestablished Uu bearers.

The UE 104 accesses, at step S1206, the MCPTT service 106 using IMS/SIPprocedures after mutual authentication and establishment of secureassociation (SA-R) between UE 104 and the MCPTT server 106. Uponsuccessfully establishing the Uu bearers, the application data flows maybe switched to the established Uu bearers. If there are no more UEs 104employing relaying operation, the UNR 102 may stop its relayingactivity, at step S1207, and send a UNR Mode Stop indication to itsserving eNB 108. It should be noted that the serving eNB of the UNR 102may not be the same eNB as the serving eNB of the UE 104.

UE Conditionally Switches to NMO (e.g., NMO-R Service Deteriorating,Explicit Signaling From Network or the UNR, etc.)

Triggering Conditions for Switching to NMO Mode

In this instance, the out of coverage UE 104 does not switch to NMOautomatically or unconditionally upon finding network coverage. Instead,the UE 104 continues operating in NMO-R until a trigger causes the UE104 to switch to NMO-R. Examples of such conditions include radio linkdegradation on the PC5 interface, which may include degradation of thePC5 link quality or loss of synchronization on PC5 link, etc. Anothercondition may be service quality degradation where an applicationdetects that the quality of the received service has degraded below apredetermined threshold. For instance, this service quality may includethe detection of a predetermined number or percentage ofmissed/un-decoded voice frames or video frames, determination that theresidual bit error rate on the application packets has exceeded apredetermined threshold, etc. Another condition may be the servicebecoming unavailable such that the UNR 102 no longer supports theservice the UE 104 is interested in (e.g., due to lack of PC5 resources,etc.). Other UNR related parameters indicating deterioration could alsotrigger switching, such as a low battery level reported by the UNR 102or other explicit messages received from the UNR 102 necessitating aswitch to NMO. Examples of such explicit messages may include commandsindicating UNR mode termination of the relay or capacity of the relayexceeded, etc.

Switching to NMO

The flowchart 1300 of FIG. 13 illustrates an example procedure fortriggering the switch to NMO. The UE 104 begins, at block S1301, inNMO-R. If a suitable eNB 108 capable of supporting the UE 104 during NMOoperation is found, at block S1302, and the trigger conditions for theswitch to NMO are met, at block S1303, the UE 104 initiates, at blockS1304, mechanisms to switch from NMO-R to NMO.

The procedure for the UE 104 to switch to NMO mode of operation isdetailed in FIG. 14 and is somewhat similar to the procedures depictedin FIGS. 11 and 12. The UE 104 operating in NMO-R through the UNR 102enters network coverage, at step S1401, and selects a suitable cell. TheUE 104 determines, at step S1402, that conditions to switch to NMO aresatisfied as described above. The UE 104 establishes, at step S1403, anRRC connection in order to get MCPTT service in the serving cell (if theUE 104 has not already switched to RRC_CONNECTED for other reasons suchas being paged by the network for a mobile terminating session, etc.)and establishes the PDN connection(s) employed by the services carriedover PC5 interface. The service request procedure is initiated by a NASlayer to establish the necessary EPS bearers for NMO.

The UE 104 accesses, at step S1404, the MCPTT service 106 using IMS/SIPprocedures after mutual authentication and establishment of secureassociation (SA-R) between UE 104 and the MCPTT server 106. Uponsuccessfully establishing the Uu bearers, the application data flows maybe switched to the established Uu bearers. If needed, (e.g., notprevented by the MCPTT server 106), the UE 104 may suppress duplicateinformation that could be temporarily received from the relay and fromthe network.

The UE 104 sends, at step S1405, a Sidelink Disconnect indication to theUNR 102 to stop the relay transferring MCPTT information for this UE104. At step S1406, the UNR 102 stops transmissions directed towards theUE 104. The UNR 102 may answer, at step S1407, the Sidelink Disconnectindication by a Sidelink Disconnect Ack. If there are no more UEs 104employing relaying operation, the UNR 102 may stop its relayingactivity, at step S1408, and send a UNR Mode Stop indication to theserving eNB 108.

Again, if the UE 104 is not capable of supporting NMO-R whilst inRRC_CONNECTED state, then a Break-Before-Make solution would be used asdepicted in FIG. 15. The UE 104, operating in NMO-R through the UNR 102,enters network coverage and selects a suitable cell, at step S1501. TheUE 104 determines, at step S1502, that conditions to switch to NMO aresatisfied as described above. The UE 104 sends, at step S1503, aSidelink Disconnect indication to the UNR 102 to stop the relaytransferring MCPTT information for this UE 104. The UNR 102 stopstransmissions directed towards the UE 104, at step S1504. The UNR 102may answer, at step S1505, the Sidelink Disconnect indication by aSidelink Disconnect Ack. If there are no more UEs 104 employing relayingoperation, the UNR 102 may stop its relaying activity, at step S1506,and send a UNR Mode Stop indication to the serving eNB 108. The UE 104establishes an RRC connection in step S1507 in order to get MCPTTservice in the serving cell (if the UE has not previously switched toRRC_CONNECTED for other reasons) and establish PDN connection(s)required by the services provided over PC5. The UE 104 accesses theMCPTT service, at step S1508, using IMS/SIP procedures after mutualauthentication and establishment of secure association (SA-R) between UE104 the MCPTT server 106.

The Equipment

A block diagram of an example of a wireless communication device 1600(such as UE 104 and UNR 102) is shown in FIG. 16. The wirelesscommunication device 1600 includes multiple components, such as aprocessor 1602 that controls the overall operation of the wirelesscommunication device. Communication functions, including data and voicecommunications, are performed through a communication subsystem 1604.The communication subsystem 1604 may include a plurality of receiversand transmitters operating on one or more frequencies to allowsimultaneous connection to two or more different entities. For UEshaving MBB capabilities, at least two receivers and two transmitters maybe employed. Data received by the wireless communication device isdecompressed and decrypted by a decoder 1606. The communicationsubsystem 1604 receives messages from and sends messages to a wirelessnetwork 1650. The wireless network 1650 may be any type of wirelessnetwork, including, but not limited to, data wireless networks, voicewireless networks, and networks that support both voice and datacommunications.

A power source 1642, such as one or more rechargeable batteries or aport to an external power supply, powers the wireless communicationdevice 1600.

The processor 1602 interacts with other components, such as RandomAccess Memory (RAM) 1608, memory 1610, a display 1612 (which may be atouch-sensitive display), one or more actuators 1620, an auxiliaryinput/output (I/O) subsystem 1624, a data port 1626, a speaker 1628, amicrophone 1630, short-range communications subsystem 1632, and otherdevice subsystems 1634. User-interaction with a graphical user interfaceis performed through the touch-sensitive display 1612. Information, suchas text, characters, symbols, images, icons, and other items that may bedisplayed or rendered on a portable electronic device, is displayed onthe touch-sensitive display 1612 via the processor 1602. The processor1602 may interact with an accelerometer 1636 that may be utilized todetect direction of gravitational forces or gravity-induced reactionforces.

To identify a subscriber for network access, the wireless communicationdevice 1600 uses a UICC such as a Subscriber Identity Module or aRemovable User Identity Module (SIM/RUIM) card 1638 for communicationwith a network, such as the wireless network 1650. Alternatively, useridentification information may be programmed into memory 1610.

The wireless communication device 1600 includes an operating system 1646and software programs or components 1648, such as the MCPTT application1644, that are executed by the processor 1602 and are typically storedin a persistent, updatable store such as the memory 1610. Additionalapplications or programs may be loaded onto the wireless communicationdevice 102, 104 through the wireless network 1650, the auxiliary I/Osubsystem 1624, the data port 1626, the short-range communicationssubsystem 1632, or any other suitable subsystem 1634.

A received signal such as a text message, an e-mail message, instantmessage or web page download is processed by the communication subsystem1604 and input to the processor 1602. The processor 1602 processes thereceived signal for output to the display 1612 and/or to the auxiliaryI/O subsystem 1624. A subscriber may generate data items, for examplee-mail messages, which may be transmitted over the wireless network 1650through the communication subsystem 1604. For voice communications, theoverall operation of wireless communication device 102, 104 is similar.The speaker 1628 outputs audible information converted from electricalsignals, and the microphone 1630 converts audible information intoelectrical signals for processing.

The touch-sensitive display 1612 may be any suitable touch-sensitivedisplay, such as a capacitive, resistive, infrared, surface acousticwave (SAW) touch-sensitive display, strain gauge, optical imaging,dispersive signal technology, acoustic pulse recognition, and so forth,as known in the art. A capacitive touch-sensitive display includes acapacitive touch-sensitive overlay. The overlay may be an assembly ofmultiple layers in a stack including, for example, a substrate, a groundshield layer, a barrier layer, one or more capacitive touch sensorlayers separated by a substrate or other barrier, and a cover. Thecapacitive touch sensor layers may be any suitable material, such aspatterned indium tin oxide (ITO).

One or more actuators 1620 may be depressed or activated by applyingsufficient force to the actuators 1620 to overcome the actuation forceof the actuator. The actuator(s) 1620 may provide input to the processor1602 when actuated. Actuation of the actuator(s) 1620 may result inprovision of tactile feedback.

Turning now to FIG. 17, a block diagram of an example eNB 108 isprovided. The eNB 108 includes at least one processor 1702 that controlsthe overall operation of the eNB 108. Wired communication subsystem 1704allows the eNB 108 to interact with various other devices, such asservers (e.g., an MCPTT application server), routers, gateways, etc.,via a wired network such as the Internet. Wireless communicationfunctions, including data and voice communications, are performedthrough a wireless communication subsystem 1706.

The eNB 108 includes memory 1708 storing computer-readable instructionsfor an operating system 1710, data 1712 and software programs orcomponents 1714 that are executed by the processor 1702. It should benoted that other typical functionality and components of an eNB 108 arenot shown here for simplicity and brevity.

Aspects of the present disclosure may be embodied as a device orapparatus, system, method or computer program product. Accordingly,aspects of the present disclosure may take the form of an entirelyhardware-based embodiment, an entirely software-based embodiment(including firmware, resident software, micro-code, etc.) or anembodiment combining software and hardware that may all generally bereferred to herein as a “circuit,” “module” or “system.” Furthermore,aspects of the present disclosure may take the form of a computerprogram product embodied in one or more computer readable medium(s)having computer readable program code embodied thereon. A computerreadable storage medium may be, for example, but not limited to, anelectronic, magnetic, optical, electromagnetic, infrared, orsemiconductor system, apparatus, or device, or any suitable combinationof the foregoing. More specific examples (a non-exhaustive list) mayinclude the following tangible media: an electrical connection havingone or more wires, a portable computer diskette, a hard disk, a randomaccess memory (RAM), a read-only memory (ROM), an erasable programmableread-only memory (EPROM or Flash memory), an optical fiber, a portablecompact disc read-only memory (CD-ROM), an optical storage device, amagnetic storage device, or any suitable combination of the foregoing.Non-tangible or non-transitory media may include a propagated datasignal with computer readable program code embodied therein, forexample, in baseband or as part of a carrier wave. Such a propagatedsignal may take any of a variety of forms, including, but not limitedto, electro-magnetic, optical, or any suitable combination thereof.Computer program code or instructions for carrying out operations foraspects of the present disclosure may be any combination of one or moreprogramming languages, including an object oriented programming languageand conventional procedural programming languages. The program code mayexecute on one or more devices such as a computer and/or server.

Aspects of the present disclosure have been described above withreference to flowchart illustrations and/or block diagrams of methods,apparatus (systems) and computer program products according toembodiments of the disclosure. In this regard, the flowchart and blockdiagrams in the figures illustrate the architecture, functionality, andoperation of possible implementations of systems, methods and computerprogram products according to various embodiments. However it shouldalso be noted that, in some alternative implementations, the functionsnoted in the block may occur out of the order noted in the figures. Forexample, two blocks shown in succession may, in fact, be executedsubstantially concurrently, or the blocks may sometimes be executed inthe reverse order, depending upon the functionality involved. It willalso be noted that each block of the block diagrams and/or flowchartillustration, and combinations of blocks in the block diagrams and/orflowchart illustration, can be implemented wholly or partially byspecial purpose hardware-based systems that perform the specifiedfunctions or acts, or combinations of special purpose hardware andcomputer instructions. Furthermore it also will be understood that eachblock of the flowchart illustrations and/or block diagrams, andcombinations of blocks in the flowchart illustrations and/or blockdiagrams, can be implemented wholly or partially by computer programinstructions. These computer program instructions may be provided to aprocessor of a general purpose computer, special purpose computer, orother programmable data processing apparatus to produce a machine, suchthat the instructions, which execute via the processor of the computeror other programmable data processing apparatus, create means forimplementing the functions/acts specified in the flowchart and/or blockdiagram block or blocks.

Finally, the terminology used herein is for the purpose of describingparticular embodiments only and is not intended to be limiting. That is,the description of the present disclosure has been presented forpurposes of illustration and description, but is not intended to beexhaustive or limited to the form disclosed. Many modifications andvariations will be apparent without departing from the scope of thedisclosure defined in the appended claims.

We claim:
 1. A method of switching between modes of operation for adevice in a cellular network comprising: accessing a service via aconnection with a serving cell; detecting, while accessing the servicevia the connection with the serving cell, an absence of a suitableneighbor cell and a condition indicative of a deterioration in theservice accessed via the connection with the serving cell; andinitiating discovery of relay nodes responsive to detecting both thecondition indicative of a deterioration in the service accessed via theconnection with the serving cell and the absence of a suitable neighborcell, wherein the absence of a suitable neighbor cell occurs when areceived power level from every neighbor cell is less than apredetermined threshold, or when a received signal quality from everyneighbor cell is less than a predetermined threshold.
 2. The method ofclaim 1, wherein the service is a mission critical push-to-talk overLong Term Evolution service.
 3. The method of claim 1, wherein a radiolink degradation on an air-interface between the device and the servingcell is the condition indicative of a deterioration in the serviceaccessed via the connection with the serving cell.
 4. The method ofclaim 1, wherein an anticipation of an imminent radio link failure isthe condition indicative of a deterioration in the service accessed viathe connection with the serving cell.
 5. The method of claim 1, whereinthe condition indicative of a deterioration in the service accessed viathe connection with the serving cell is an application service qualitydegradation below a predetermined threshold.
 6. The method of claim 5,wherein the application service quality degradation includes exceeding apredetermined number of missed data frames.
 7. The method of claim 1,wherein the condition indicative of a deterioration in the serviceaccessed via the connection with the serving cell is the servicebecoming unavailable.
 8. The method of claim 1, wherein the absence of asuitable neighbor cell occurs when the received power level from everyneighbor cell is less than the predetermined threshold.
 9. The method ofclaim 1, wherein the absence of a suitable neighbor cell occurs when thereceived signal quality from every neighbor cell is less than thepredetermined threshold.
 10. The method of claim 1, further comprising:subsequent to initiating discovery of relay nodes, discovering asuitable relay node; and initiating a mechanism, after the discovering,to switch to access of the service via the suitable relay node ratherthan the serving cell.
 11. The method of claim 10, wherein the suitablerelay node is a user equipment capable of acting as a userequipment-to-network relay node.
 12. The method of claim 10, wherein thesuitable relay node is capable of relaying the service. 13.Non-transitory computer-readable storage media storing code which, whenexecuted by a processor of a communication system, causes thecommunication system to: access a service via a connection with aserving cell; detect, while accessing the service via the connectionwith the serving cell, an absence of a suitable neighbor cell and acondition indicative of a deterioration in the service accessed via theconnection with the serving cell; and initiate discovery of relay nodesresponsive to detecting both the condition indicative of a deteriorationin the service accessed via the connection with the serving cell and theabsence of a suitable neighbor cell, wherein the absence of a suitableneighbor cell occurs when a received power level from every neighborcell is less than a predetermined threshold, or when a received signalquality from every neighbor cell is less than a predetermined threshold.14. The non-transitory computer-readable storage media of claim 13,wherein the service is a mission critical push-to-talk over Long TermEvolution service.
 15. The non-transitory computer-readable storagemedia of claim 13, wherein the condition indicative of a deteriorationin the service accessed via the connection with the serving cell is anapplication service quality degradation exceeding a predetermined numberof missed data frames.
 16. The non-transitory computer-readable storagemedia of claim 13, wherein the communication system is further causedto: subsequent to initiating discovery of relay nodes, discover asuitable relay node; and initiate a mechanism, after the discovering, toswitch to access of the service via the suitable relay node rather thanthe serving cell.
 17. A communication device comprising: at least onecommunication subsystem; and a processor configured to: access a servicevia a connection with a serving cell; detect, while accessing theservice via the connection with the serving cell, an absence of asuitable neighbor cell and a condition indicative of a deterioration inthe service accessed via the connection with the serving cell; andinitiate discovery of relay nodes responsive to detecting both thecondition indicative of a deterioration in the service accessed via theconnection with the serving cell and the absence of a suitable neighborcell, wherein the absence of a suitable neighbor cell occurs when areceived power level from every neighbor cell is less than apredetermined threshold, or when a received signal quality from everyneighbor cell is less than a predetermined threshold.
 18. Thecommunication device of claim 17, wherein the communication system isfurther caused to: subsequent to initiating discovery of relay nodes,discover a suitable relay node; and initiate a mechanism, after thediscovering, to switch to access of the service via the suitable relaynode rather than the serving cell.
 19. The communication device of claim17, wherein the condition indicative of a deterioration in the serviceaccessed via the connection with the serving cell is an applicationservice quality degradation exceeding a predetermined number of misseddata frames.